1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to wireless communication technology, and more particularly, to a wirelessly transmitting large data in a more efficient and stable manner.
2. Description of the Related Art
As networks become wireless and the demand for large multimedia data transmission increases, there is a need for studies on an effective transmission method in a wireless network environment. In a wireless network, a plurality of devices share given wireless resources. Therefore, if contention increases, there is a high possibility of losing valuable wireless resources due to collisions during communication. In order to reduce such collisions or losses and facilitate secure data transmission/reception, a contention-based distributed coordination function (DCF) or a contention-free point coordination function (PCF) is used in a wireless local area network (LAN) environment, and a time division method, such as channel time allocation, is used in a wireless personal area network (PAN) environment.
By applying these methods to a wireless network, collisions can be reduced to a certain degree, and stable communication can be achieved. However, the wireless network still has a greater possibility of collision between transmission data than a wired network. This is because a lot of factors that hinder stable communication, such as multi-path, fading and interference, are inherent in the wireless network environment. In addition, as the number of wireless devices that join the wireless network increases, problems, such as collisions and losses, are more likely to occur.
The collisions require retransmissions, which severely undermine the throughput of the wireless network. In particular, when a better quality of service (QoS) is required as in the case of audio/video (AV) data, it is very important to secure available bandwidth as much as possible by reducing the number of retransmissions.
Considering that various home devices are increasingly required to wirelessly transmit high-quality videos, such as digital video disk (DVD) videos or high definition television (HDTV) videos, it is time to develop a technological standard for seamlessly and consistently transmitting or receiving high-quality videos that require broad bandwidth.
An IEEE 802.15.3c task group is developing a technological standard for transmitting large-volume data over a wireless home network. The technological standard, which is called “millimeter wave (mmWave),” uses an electric wave having a physical wavelength of a millimeter (i.e., an electric wave having a frequency band of 30-300 GHz) to transmit large-volume data. This frequency band, which is an unlicensed band, has conventionally been used by communication service providers or used for limited purposes, such as observing electric waves or preventing vehicle collision.
FIG. 1 is a diagram comparing frequency bands of IEEE 802.11 series of standards and mmWave. Referring to FIG. 1, an IEEE 802.11b or IEEE 802.11g standard uses a carrier frequency of 2.4 GHz and has a channel bandwidth of approximately 20 MHz. In addition, an IEEE 802.11a or IEEE 802.11n standard uses a carrier frequency of 5 GHz and has a channel bandwidth of approximately 20 MHz. On the other hand, mmWave uses a carrier frequency of 60 GHz and has a channel bandwidth of approximately 0.5-2.5 GHz. Therefore, it can be understood that mmWave has a far greater carrier frequency and channel bandwidth than the conventional IEEE 802.11 series of standards.
When a high-frequency signal (a millimeter wave) having a millimeter wavelength is used, a very high transmission rate of several Gbps can be achieved. Since the size of an antenna can also be reduced to less than 1.5 mm, a single chip including the antenna can be implemented. Furthermore, interference between devices can be reduced due to a very high attenuation ratio of the high-frequency signal in the air.
However, the high-frequency signal has a short distance range due to the very high attenuation ratio. In addition, since the high-frequency signal is highly directional, it is difficult to have a proper communication in a non-line-of-sight environment. In mmWave, an array antenna having a high gain is used to solve the former problem, and a beam steering method is used to solve the latter problem.
Recently, a method of transmitting uncompressed data using mmWave in a high-frequency band of several tens of GHz has been introduced to home and office environments, along with a conventional method of transmitting compressed data using a band of several GHz of IEEE 802.11 Standards.
Since uncompressed AV data is large-volume data that is not compressed, it can be transmitted only in a high-frequency band of several tens of GHz. Even when having a packet loss, uncompressed AV data has relatively less effect on the quality of displayed video than compressed data. Therefore, there is no need for an automatic repeat request or a retry. In this regard, an efficient medium access method is required to efficiently transmit uncompressed AV data in a high frequency of several tens of GHz.